Here is the original patent info from Ten-Tec for their variable bandwidth filter.
This method has been used by them , Elecraft and others in multiple designs.It is always going to be somewhat of a compromise compared to separate filters for different bandwidths. The better designs have individual adjustments to tune the shape of the filter for the best overall response.
http://www.google.com/patents/US5051711A search on 'variable bandwidth crystal filter' gives quite a few different articles on them.
DuWayne Kv4QB On 3/20/2014 4:45 PM, Joe Rocci wrote:
I recall that someone actually swept that filter under different
settings (or it was possibly an Elecraft rig that uses the same
technique) and verified that the shape and loss do indeed suffer. If
someone could find a way to compensate the impedance match over the
tuning range from CW bandwidth to phone bandwidth, it would be killer.
You could program a PWM in the AVR controller to produce a variable
output voltage, resulting in software controlled bandwidth selection.
----- Original Message -----
*From:* Lawrence Galea <mailto:9h1avlaw@xxxxxxxxx>
*To:* minima@xxxxxxxxxxxxx <mailto:minima@xxxxxxxxxxxxx>
*Sent:* Thursday, March 20, 2014 4:09 PM
*Subject:* [minima] Re: Rx only minima for swl
You can have a look at the circuit of the multipig xcvr where
instead of capacitors in the crystal filter it uses varicaps and
by changing the voltage across the varicaps the filter bandwidth
is changed.
http://kitsandparts.com/rcvr.gif
Lawrence
On Thu, Mar 20, 2014 at 7:47 PM, Joe Rocci <joe@xxxxxxxxxx
<mailto:joe@xxxxxxxxxx>> wrote:
"Use relays or active device switches to decrease capacitor values
in the crystal filter"
Arv
Keep in mind that anything you do to change the filter
bandwidth will also change the characteristic impedance.
Impedance mismatch in bandpass filters usually results in
distorted passband shape and increased loss. I think the
second approach is better.
Joe
W3JDR
----- Original Message -----
*From:* Arv Evans <mailto:arvid.evans@xxxxxxxxx>
*To:* minima@xxxxxxxxxxxxx <mailto:minima@xxxxxxxxxxxxx>
*Sent:* Thursday, March 20, 2014 2:31 PM
*Subject:* [minima] Re: Rx only minima for swl
Cor
It really depends on whether one wants a general coverage
communications
quality receiver or a broadcast quality receiver. If
wider IF bandwidth is
needed then there are at least two approaches to achieving it.
* Use relays or active device switches to decrease
capacitor values
in the crystal filter.
* Add a second wider bandwidth IF filter and switch the
filter in
or out using relays or active device switches.
Changing bandwidth of the IF filter also could be applied
for CW or other
narrow bandwidth modes.
Arv K7HKL
_._
On Thu, Mar 20, 2014 at 12:22 PM, dx11 <dx11@xxxxxxxxx
<mailto:dx11@xxxxxxxxx>> wrote:
It crossed my mind as well to make a Rx only version
with AM added. I did not yet come to a solution how
to widen the Xtal filter to an AM value.
Cor Beijersbergen
It appears from a quick look at the schematic that you
should be able to
add an AM detector and switch the audio amplifier
input between outputs
of SSB and AM detectors. To avoid interference you
might want to switch
the BFO oscillator power off when listening to AM.
I like your idea of using this design as a general
coverage receiver. May
have to try that myself.
Arv K7HKL
_._
-- Of all the things I lost, I miss my mind the most.
